(1) Field of the Invention
The present invention relates to an optical transmitting apparatus and an optical transmitting system. Particularly, the present invention relates to a technique for optimizing signal quality of an output optical signal of an optical amplifier in an optical transmitting apparatus or an optical transmitting system having the optical amplifier in a hybrid structure.
(2) Description of Related Art
With an increase in information communication quantity, development of optical fiber communication systems in large capacities and at low costs is lively in these years. For the purpose of increasing the capacity, there has been studied and developed a wavelength division multiplex (WDM: Wavelength Division Multiplex) transmission system which multiplexes optical signals at a plurality of wavelengths as a channel signal and transmits it. To realize a lower cost, there is a demand for a long-distance system, in which a distance (called a 3R section) between a terminal (terminal once converting an optical signal into an electric signal, and again regenerating the optical signal) to a terminal can be extended as much as possible.
In the latter, in order to extend the 3R section as much as possible, it is essential how low an optical noise level generating in an optical amplifier disposed in a regenerator can be suppressed. With regard to this point, distribution Raman amplifiers (DRAs) attract attention in recent years, which are tried to be introduced into real systems.
Optical signals are heretofore amplified intensively by an erbium doped fiber amplifier (EDFA) in a regenerator. However, the EDFA generates a relatively large amount of noise, although being able to amplify with a high gain, which is one of factors that limit a transmission distance of the whole system. Namely, even if an optical receiving terminal can receive an optical signal at a sufficient level (power) transmitted for a long distance, the terminal cannot normally demodulate the signal because of a poor optical signal to noise ratio (OSNR) representing quality of the received optical signal.
In order to avoid such phenomenon, a DRA is disposed in front of the EDFA, for example, as a structure of an optical amplifier (hereinafter referred as an amplifier structure) of a regenerator, a part of a transmission loss due to an optical transmission path (optical fiber) is compensated by the DRA, then the EDFA intensively amplifies the optical signal.
Advantage of this amplifier structure is that this amplifier structure is expected to improve the OSNR as compared with a system configured with only the EDFA, since the DRA can amplify with a lower noise than that of the EDFA because the DRA is an optical amplifier of a distribution amplifier type which distributively amplifies an optical signal using the optical transmission path although its gain is smaller than that of the EDFA.
FIG. 11 shows an example of WDM transmission system using a hybrid amplifier configured with a DRA and an EDFA as a regenerator. In the WDM transmission system shown in FIG. 11, optical signals each at a predetermined wavelength are generated by respecitve transponders (optical transmitters) 101 equal in number to multiplexed wavelengths, signal levels of the optical signals are adjusted by respective optical variable attenuators 102 each at a corresponding wavelength, and the optical signals are wavelength-multiplexed by an optical multiplexer 103 in an optical transmitting terminal 101, then sent as a WDM signal to an optical transmission path 500.
The WDM signal is transmitted to an optical receiving terminal 400 while being amplified by regenerators 200 and 300. Namely, a DRA 301 complimentarily amplifies the WDM signal with a low noise by distribution amplification to compensate a part of a transmission loss due to the optical transmission path 500, and EDFAs 201 and 302 intensively amplify the WDM signal to compensate a remaining transmission loss due to the opical transmission path 500, whereby the WDM signal is transmitted through the optical transmission path 500. Incidentally, the number of the regenerators 200 and 300 is determined according to a transmission distance between the optical transmission terminal 100 and the optical receiving terminal 400, and regenerating (amplifying) capacities of the regenerators 200 and 300.
When the above WDM signal is finally received by the optical receiving terminal 400, the WDM signal is demultiplexed into optical signals at respective wavelengths by an optical demultiplexer 401, and received by optical receivers 402 at respective wavelengths.
In the case of a hybrid optical amplifier using the DRA 301 and the EDFA 302 as described above, it is possible to arbitrarily set a set gain of the DRA 301 and a set gain of the EDFA 302 within certain ranges (input level operable ranges of the DRA 301 and the EDFA 302) by changing pumping conditions of them.
Generally, there is used a method of estimating a DRA gain fluctuation due to variations in transmission path loss immediately before the DRA 301 and variations in fiber parameters of the optical transmission path 500, and beforehand setting a pumping light power of the DRA 301 so that a range of the DRA gain fluctuation falls in the operating range (input dynamic range of the EDFA 302) of the EDFA 302.
An output level of the regenerator (hereinafter referred as a node) 200 or 300 is defined in consideration of a non-linear phenomenon generating on the transmission path, so that outputs of the EDFA 302 are required to be kept at a constant level (power). For this, as disclosed in Japanese Patent Laid-Open Publication No. 2000-98433, for example, there is proposed a technique (hereinafter referred as a known technique) of monitoring a level (power) of an output optical signal of the DRA or EDFA, feed-back-controlling a pumping light power of the DRA such that a level of the output optical signal (level of an input optical signal to the EDFA) is always constant, thereby controlling an output of the regenerator at a constant level.
However, operating conditions of the DRA 301 are determined by fiber parameters of the optical transmission path 500 in the above method of beforehand setting the pumping light power of the DRA 301. When considering characteristics of a total node of the DRA 301 plus EDFA 302, it is difficult to say that they operate in the optimum conditions from a viewpoint of OSNR.
The above described known technique is to feed-back-control the pumping light power of the DRA in order to control a level (power) of outputs of the regenerator at a defined constant level, which does not improve the OSNA. Accordingly, it is hardly said that the regenerator operates under conditions that the optimum OSNR of the whole node can be obtained, like the above pre-setting method.
In a WDM transmission system, a dispersion compensating fiber (DCF: Dispersion Compensating Fiber) is generally installed in a node, when it is necessary to compensate wavelength dispersion generating in a WDM signal due to a wavelength dependent transmission loss characteristic of the optical transmission path 500. As a position at which the dispersion compensating fiber is to be disposed, it is said that between stages of the EDFAs (structure in which the EDFAs are in two stages, and the DCF is interposed between the stages) is suitable. This is to prevent degradation of the OSNR due to a DCF loss as much as possible by interposing the DCF having a relatively large loss between the stages of the EDFAs.
In a hybrid optical amplifier structure, there is used a DRA with an optical fiber being as an amplification medium. As compared with a known node structure including only an EDFA, a level diagram of an optical signal in the node is largely different, so that it is not always optimum from a viewpoint of OSNR that the DCF is interposed between stages of the EDFA.
In the light of the above problems, and object of the present invention is to optimize optical signal quality (OSNR) in an optical transmitting apparatus in a hybrid optical amplifier structure.
To attain the above object, the present invention provides an optical transmitting apparatus comprising a first optical amplifier by which quality of an output optical signal after amplified is changed according to its amplification gain, a second optical amplifier by which quality of an output optical signal after amplified is changed according to an input level of the output optical signal from the first optical amplifier, and a controlling means for performing an adaptive control on the amplification gain of the first optical amplifier so that quality of the output optical signal outputted from the second optical amplifier becomes maximum.
In the optical transmitting apparatus according this invention structured as above, the above controlling means so controls an amplification gain of the first optical amplifier that output optical signal quality of the second optical amplifier becomes maximum. Accordingly, it is possible to optimize final quality of an output optical signal of the optical transmitting apparatus although an input level of an optical signal to the second optical amplifier is not always constant unlike a case where an amplification gain of the first optical amplifier is so controlled that an output optical signal level of the second optical amplifier is constant.
The above controlling means may comprise a control target value storing unit for storing an output optical signal level of the first optical amplifier, at which quality of the output optical signal of the second optical amplifier is maximum, beforehand determined as a control target value on the basis of a gain-to-noise characteristic of the first optical amplifier and an input level-to-noise characteristic of the second optical amplifier, and a first gain control unit for setting the amplification gain of the first optical amplifier on the basis of the control target value of the control target value storing unit.
By employing the above structure, the controlling means can realize optimization of output optical signal quality of the whole optical transmitting apparatus, using a simple control that an amplification gain of the first optical amplifier is set on the basis of the above control target value beforehand determined on the basis of a noise characteristic of each of the above optical amplifiers.
In this case, the above first gain control unit may comprise a level monitoring unit for monitoring an output optical signal level of the first optical amplifier, and a comparing unit for comparing the control target value of the control target value storing unit with an output optical signal level of the first optical amplifier monitored by the level monitoring unit, thereby controlling the amplification gain of the first optical amplifier so that a result of comparison by the comparing unit becomes minimum. Accordingly, even after the above amplification gain setting, it is possible to always maintain the output optical signal level of the first optical amplifier at an optimum value at which output optical signal quality of the second optical amplifier is maximum.
The above controlling means may comprise a control target value calculating mans for calculating the control target values for each of second optical amplifiers having a different input level-to-noise characteristic on the basis of the gain-to-noise characteristic of the first optical amplifier and the input level-to-noise characteristic of the second optical amplifier, and storing the control target values in the control target value storing unit. In this case, an amplification gain of the first optical amplifier is always controlled on the basis of quality of an actual output optical signal of the second optical amplifier, so that it is possible to optimize output optical signal quality of the whole transmitting apparatus even at a time of initial setting of an amplification gain of the first optical amplifier and in an operating state after the initial setting.
In an optical transmitting system according to the present invention, an optical regenerator comprises a first optical amplifier by which quality of an output optical signal after amplified is changed according to its amplification gain, and a second optical amplifier by which quality of an output optical signal after amplified is changed according to an input level of the output optical signal from the first optical amplifier, whereas an optical receiver comprises an optical signal quality monitoring unit for monitoring quality of a received optical signal, and a control unit for controlling the amplification gain of the first optical amplifier in the optical regenerator so that quality of the received optical signal monitored by the optical signal quality monitoring unit becomes maximum.
In the optical transmitting system according to this invention structured as above, an amplification gain of the first optical amplifier in the optical regenerator in the system is so controlled that quality of a received optical signal monitored in the optical receiver becomes maximum. Accordingly, it is possible to optimize quality of a received signal in the optical receiver, that is, quality of an optical signal of the whole transmitting system, without providing the above adaptive control function to each regenerator in the system.
The optical transmitting apparatus and the optical transmitting system according to this invention provide the following effects and advantages:
(1) An amplification gain of the first optical amplifier is not such controlled that a level of an output optical signal of the second optical amplifier is always constant, but such adaptively controlled that quality of an output optical signal outputted from the second optical amplifier becomes maximum. Accordingly, quality of an output optical signal of the whole transmitting apparatus can be in the bast state. This allows a transmission distance of an optical signal to be extended, and the optical transmitting system to be configured at a low cost.
(2) The above adaptive control can be realized by a simple control that an amplification gain of the first optical amplifier is set on the basis of a control target value beforehand determined on the basis of a noise characteristic of each of the optical amplifiers. Accordingly, it is possible to realize, in a simple structure, a gain control by which quality of an optical signal of the whole optical transmitting apparatus can be always optimized. This allows a reduction in size and cost of the controlling means, further of the optical transmitting apparatus.
(3) An output optical signal level of the first optical amplifier is monitored, the output optical signal level is compared with the above control target value, and an amplification gain of the first optical amplifier is such controlled that a result of the comparison is minimum. It is thereby possible to always maintain quality of an optical signal of the whole optical transmitting apparatus in the optimum state, thus the whole optical transmitting system can be operated in the best conditions with respect to the optical signal quality.
(4) The above control target value can be set to each of second optical amplifiers having different noise characteristics on the basis of a gain-to-noise characteristic of the first optical amplifier and an input level-to-noise characteristic of the second optical amplifier. Even when another second optical amplifier having a different noise characteristic is employed in the optical transmitting apparatus, it is possible to automatically optimize output optical signal quality of the whole optical transmitting apparatus, which largely contributes to improvement of flexibility in configuring the system.
(5) The above adaptive control can be realized by monitoring quality of an output optical signal of the second optical amplifier, and such controlling an amplification gain of the second optical amplifier that the quality becomes maximum. In this case, an amplification gain of the first optical amplifier is always controlled on the basis of actual quality of an output optical signal of the second optical signal. Accordingly, even at a time of initial setting of an amplification gain of the first optical amplifier and in an operating state after the initial setting, it is possible to optimize output optical signal quality of the whole optical transmitting apparatus.
(6) The adaptive control in this case can be realized by monitoring, for example, an error rate of an output optical signal of the second optical amplifier, and such controlling an amplification gain of the first optical amplifier that the error rate becomes minimum. In this case, it is possible to always reduce the signal error rate of the whole optical transmitting apparatus to the minimum, and optimize signal quality of the whole transmitting apparatus, as well.
(7) The above adaptive control can be realized by monitoring an optical signal-to-noise ratio of an output optical signal of the second optical amplifier by an optical spectrum analyzer or the like, and such controlling an amplification gain of the first optical amplifier that an actual optical signal-to-noise ratio becomes maximum. In this case, it is possible to always operate the optical transmitting apparatus in conditions that signal quality (optical signal-to-noise ratio) of an output optical signal of the whole optical transmitting apparatus is best.
(8) Further, the dispersion compensator for compensate wavelength dispersion of a wavelength-multiplexed optical signal is disposed at a position, at which quality of an output optical signal of the second optical amplifier is maximum, beforehand determined on the basis of a noise characteristic of each of the optical amplifiers, among a position in a front stage of the first optical amplifier, a position between the first optical amplifier and the second optical amplifier, and a position in a rear stage of the second optical amplifier. Even when it is necessary to compensate wavelength dispersion in the optical transmitting apparatus, wavelength dispersion can be compensated at an appropriate position from a viewpoint of output optical signal quality in the whole optical transmitting apparatus, and the optical transmitting apparatus can be operated in a state that the output optical signal quality of the whole optical transmitting apparatus is best.
(9) Particularly, when the above dispersion compensator is disposed at least in a front stage of the first optical amplifier or between the first optical amplifier and the second optical amplifier, it is possible to more improve output optical signal quality of the whole optical transmitting apparatus as compared with a case where the dispersion compensator is disposed at another position.
(10) The dispersion compensator may be divided and disposed at a plurality of positions among a position in a front stage of the first optical amplifier, a position between the first optical amplifier and the second optical amplifier, and a position in a rear stage of the second optical amplifier. In which case, it is possible to allow the output optical signal quality of the whole optical transmitting apparatus to be in the best conditions.
(11) When the dispersion compensator is divided and disposed at least at a position in a front stage of the first optical amplifier and a position between the first optical amplifier and the second optical amplifier, it is possible to more improve output optical signal quality of the whole optical transmitting apparatus than a case where the dispersion compensator is disposed at another position.
(12) When a Raman optical amplifier is employed as the above first optical amplifer, whereas a rare-earth-doped optical fiber amplifier is employed as the above second optical amplifier, it is possible to obtain the above effects and advantages more effectively.
(13) In the optical receiving terminal, quality of a received optical signal is monitored, and an amplification gain of the first optical amplifier in the optical regenerator is such controlled that the quality becomes maximum, as well. It is thereby possible to operate the whole optical transmitting system in the best conditions with respect to signal quality. In this case, a gain control on the first optical amplifier is intensively performed from the optical receiving terminal. Accordingly, it is unnecessary to provide the above adaptive control to each of the regenerators, which allows simplification of the optical regenerator and reduction in cost of the same, and reduction in size, cost and the like of the whole optical transmitting system.